We are investigating several aspects of the structure and replication of enveloped RNA viruses. A major objective is to determine what differences in the amino acid sequence of related glycoproteins (G proteins) from vesicular stomatitis virus (VSV) account for the differences in their requirement for oligosaccharides. The nonglycosylated G proteins of VSV are temperature-sensitive but the degree of sensitivity depends on the amino acid sequence of the protein. Differences in amino acids will be inferred from sequencing of cDNAs derived from the mRNAs for the related glycoproteins. We are also studying defective-interfering (DI) particles of Sindbis virus and plan to continue the characterization of the defective RNA. To this end we are sequencing several DI RNAs to identify those sequences which are conserved in all DI RNAs and would be considered essential for their ability to interfere with the replication of standard virus and for them to be packaged successfully. An important goal in this work is to examine the role of both DI particles and the standard virus in establishing and maintaining persistent infections in cultured cells. In vitro models of persistent infections should lead to the development of methods for detecting low levels of virus replication and for determining the presence or absence of defective nucleic acids. An understanding of how viruses can persist in cells can provide directions to be explored in determining their putative role in disease states.